Pancreatic ductal adenocarcinoma (PDAC) is accompanied by profound systemic immunosuppression that renders this disease non-responsive to immunotherapy. One hallmark of PDAC is the dense stroma consisting of activated fibroblasts termed `pancreatic stellate cells' (PSC) that surround each tumor. Although these stromal cells produce numerous factors that may support tumor growth, recent publications also suggest a contradictory role for the stroma in protecting against metastasis. Even more surprising was the observation that depletion of PSC from genetic models of PDAC led to improved efficacy of immunotherapy. These data highlight the significance of the proposal, and how little is known regarding the intricate interactions between stroma and immune cells present within the tumor microenvironment. These results also suggest that the immune response against tumors is restrained by PSC, and that targeting key pathways in the stroma may augment the efficacy of immunotherapy. In line with these observations, a recent publication from our group demonstrated that patient-derived PSC secrete soluble factors to promote myeloid-derived suppressor cell (MDSC) differentiation. Detailed analyses revealed that STAT3 signaling was required for this process, and was due to copious amounts of interleukin-6 (IL-6) secreted from PSC. These data demonstrated a novel role for stromal PSC as a source of factors that suppress immunity in PDAC, and have fueled our interest in targeting IL-6 to enhance immunotherapy. Our preliminary data show that in vivo administration of antibodies (Ab) targeting interleukin-6 (IL-6) and PD-L1 limit tumor progression in both subcutaneous, and autochthonous, mutant KRas-driven models of PDAC. We also demonstrate this treatment combination results in increased infiltration of T cells into pancreatic tumors, and reduced levels of PSC within these same tumors. We hypothesize that stromal IL-6 is a major barrier promoting immune suppression in PDAC, and that it can be targeted to augment the response to immunotherapy. This proposal will address three Specific Aims. First, we will determine the mechanisms by which combined blockade of IL-6 and PD-L1 elicits antitumor efficacy in PDAC, focusing on phenotypic and functional properties of T cells and MDSC (Aim 1). The relative importance of PD-L1 expression on the tumor or host tissues in mediating efficacy of this treatment will be investigated. Next, we will determine if taxane-based chemotherapy augments the efficacy of IL-6 and PD-L1 blockade in autochthonous, mutant KRas-driven PDAC models (Aim 2). Finally, we will use a dual recombinase system (Flp-FRT and Cre-loxP) to develop mice with spontaneously arising, mutant KRas-driven PDAC and IL-6 deleted fibroblasts, and primary patient PSC to define the role of stromal IL-6 in promoting pancreatic cancer progression and immune suppression (Aim 3). This proposal will enhance our understanding of how the stroma influences carcinogenesis and immune suppression in PDAC. Data from these studies also have potential for near-term clinical impact with Ab targeting IL-6 in combination with immunotherapy.